212 Introduction to PBSE

The concepts of hazard and performance level are implied in the provisions of standard building codes such as International Building Code (IBC). The SEAOC Blue Book, regarded as the commentary for the Uniform Building Code (now IBC) provisions, clearly identifies the following performance objectives:

• No damage for minor levels of ground motion.

• No structural damage but some nonstructural damage for moderate ground motions.

• Structural and nonstructural damage but no collapse for major levels of ground motion (which include the strongest credible earthquake at the site).

Despite the recognition of three damage states implied in modern codes, life safety has historically been the principle concern in seismic design. Furthermore, there are no explicit procedures or processes that allow an engineer to evaluate the expected performance of the final design or assess the margin of safety provided by satisfying code requirements. Recent experience has shown that property damage and related losses resulting from minor to moderate earthquakes is significant. Nonstructural considerations and business losses can dominate the cost-benefit ratio in a life-cycle cost analysis (Krawinkler 1997). Hence, emphasis on nonstructural issues and business interruption losses must become part of the equation in a holistic treatment of seismic design. This and other concerns discussed in the previous section have led researchers and engineers to rethink the principles and process governing modern seismic design. The various alternative strategies proposed by numerous individual and group research efforts have collectively led to the evolution of performance-based concepts that allow the design of buildings (or the rehabilitation of existing buildings) with due consideration to different design objectives and varying levels of risk and loss. While current design methodology relies considerably on empirical formulations and prescriptive procedures that require only minimal design checks (such as drift limits following the sizing of sections), PBD methodology calls for a detailed demand evaluation of the building model under simulated earthquake loads to assess if the design objective has been achieved. The assessment is intended to project the potential damage and associated losses, which in turn allows all stakeholders (from the building owner to insurers and the building occupants) to specify desired levels of performance, which then becomes the basis of design for the structural engineer. While this entails a more significant responsibility on the part of the design professional, it also means that greater flexibility is possible in the design — in terms of new design alternatives and techniques (the use of seismic protection devices is one example) that can be utilized to achieve the expected performance objective.